how to compute number bytes downloaded?

[michael <linux@networkingnewsletter.org.uk>]

I'm trying to weight up whether monthly d/load caps by various ISPs
would restrict me. (Firstly I'm assuming when they say 1Gb they mean
1000*1024 bytes and not bits?) More importantly, what's the best way to
track amount of data downloaded per session? For anybody that's
interested the main thing I'll be doing is running xterms on remote
machines pointed back to mine at home but I've not idea how much traffic
X involves.

Thanks!

-- 
Michael Bane
Atmospheric Physics Group
University of Manchester